Yep, Koi are very sensitive to changes in their environment, and generally very alert to predators lurking around the pond. If the Koi in one of the tanks up here at the barn are acting skittish I can usually find the cause with a little detective work. Last Summer none of the tosai in one of the tanks came up to eat at feeding time, when there is generally a frenzy,... I knew there was an issue, and checked around the tank and found an adult water snake at the tank's edge. This happened again this morning when I came in to check the fish,... the tosai in one of the tanks didn't cime up to eat,... so I looked aroud and found this pup Raccoon up in the loft of the barn WHICH is right beside the tank with the skittish tosai. Yes, Koi will also act this way if their water has gone south,... a sudden PH crash, ammonia, oxygen depletion.
In Mike's case the Koi are likely reacting to the change in pond cover, the lack of shade,... replaced with sunlight in places within the pond they are not accustomed to seeing it, the shadows and shaded areas have changed... this in addition to the trauma of the storm itself. I would also check the pond's PH after such an influx of rain water.
The Koi that is swimming irratically MAY have some nerve, muscle, or spinal damage from the storm - heavy thunder cracks, or other loud sudden sounds which startle the fish can cause a fish to damage itself (spine) with the sudden dramatic flight response - similar to a person "throwing out" their back while doing something strenuous. Watch the Koi swim, and see if it is swimming normally,... or if it is hobbling or only flapping the tail on one side.
Best Wishes,
Brady
The visitor in the barn, taken this morning.